My biggest concern is everywhere I run is hilly as I live on the north downs, & Berlin is flat, hoping it's not going to be a problem. Going to find a flat parkrun & run it twice to see how I get on.
No, I wouldn't imagine its going to be. Actually, it might be an advantage for you. If you spent your training running up hills, you might find it easier to run (if a little boring). When I trained for London two years ago, my runs were on pathway that went gradually up hill for the first 10. Think might have helped me to improve my strength.
I also run with a running backpack, so I am training with some additional weight. This might be helpful when doing it because I will be lighter.